Crosspoint switches are well known. An article by Watson, published in EDN, Feb. 16, 1989, pages 76-86, entitled "Crosspoint-Switch IC's Enter Digital Domain" describes the crosspoint switch art and the development of digital crosspoint switches. As stated in this article, programmable crosspoint switches connect one of many input leads (also called "pads" or "terminals") to one or more output leads. While initially crosspoint integrated circuit switches were developed to replace electro-mechanical switches such as relays in electrical telephone-switching systems, crosspoint switches are now used in parallel-processing, industrial-control-routing and data-communications systems. Crosspoint switches are of two types: analog and digital. An analog crosspoint switch is nothing more than a bi-directional transmission path which passes a selected signal substantially unchanged from an input terminal to an output terminal. A digital crosspoint switch, however, is a system which will take a digital signal on one input terminal and provide a replica of that digital signal on one or more output terminals.
As disclosed in the above-cited article, all analog crosspoint switches have a number of basic elements: the switch array, an address decoder and a control memory or latches. For any N.times.M crosspoint switch, N equals the number of input terminals, M equals the number of output terminals, and the integrated circuit consists of NM distinct switches. A typical prior art analog switch (i.e., a switch which transmits passively a signal on one input or output pad to another one or more input or output pads) is a relatively inefficient device in connecting one input pad to a second input pad or one output pad to another output pad. As shown in FIG. 2 in the above-cited article, to connect, for example, the input pad X0 to the input pad X1, control signals must be applied to analog switch 0 and analog switch 1 to connect input pad X0 through analog switch 0 to the output pad Y0 and then connect the output pad Y0 through analog switch 1 to input pad X1. Accordingly, output pad Y0 is lost for other purposes. This inefficiency greatly restricts the usefulness of the analog switch of the type shown in FIG. 2.
U.S. Pat. No. 4,949,084 also discloses in FIG. 1 a cross point circuit wherein each of the input pads in circuit "A" can be connected to one or more of the output pads in circuit "B". However, should an input pad in circuit A be desired to be connected to one or more other input pads in circuit A then an output pad must be dedicated to this function.
The above crosspoint switches use decoding elements and/or multiplexers to form the desired connections. As the number of input terminals and output terminals increase, the complexity of these switches goes up extremely rapidly. Accordingly, these crosspoint switches are limited to a relatively small number of input terminals and output terminals. The total number of input and output terminals for these prior art crosspoint switches typically ranges from 8 to 128.